This invention relates to semiconductor fabrication techniques and, more particularly, to compound semiconductor devices.
It is well known that the compound semiconductor indium gallium arsenide has excellent properties that make it an attractive candidate for use in making high-speed electronic devices such as field-effect transistors (FETs). Moreover, since this semiconductor can be lattice matched to indium phosphide, it is also known that indium gallium arsenide is of practical importance for making opto-electronic integrated devices operating in the wavelength range where optical fiber loss and dispersiion are low.
The main problem that has stood in the way of widespread use of indium gallium arsenide to make microminiature FET devices is the unavailability of a suitable gate technology for this material. For example, in a metal-insulator-semiconductor FET (MISFET) in which a dielectric such as silicon dioxide overlies the indium gallium arsenide, a high and deleterious density of interface states is typically encountered. Also, in a pn-junction gate FET (JFET), it is difficult in practice to make a pn-junction gate that is one micrometer or less in length.
Schottky-barrier (SB) gate technology represents the simplest approach for making FET devices. But attempts to fabricate such metal-semiconductor FETs (MESFETs) in n-type indium gallium arsenide have heretofore not been satisfactory. This is due primarily to the relatively low-bandgap property of this semiconductor. This causes the SB height between an overlying metal and indium gallium arsenide to be so low as to result in unacceptably leaky gates.
Recently work has been reported on using an epitaxial layer of aluminum indium arsenide between the gate metal and indium gallium arsenide to enhance the SB height in a MESFET device, as described by C. K. Peng et al, "Microwave Performance of InAlAs/InGaAs/InP MODFETS", IEEE Electron, Device Lett., 1987, EDL-8, pp. 24-26. But this extra epitaxial layer makes it difficult to obtain extremely low-resistance source and drain contacts and makes the FET structure more difficult to integrate with optical devices.
Accordingly, continuing efforts have been directed by workers skilled in the art aimed at trying to devise other ways of increasing the SB height of metal-to-indium gallium arsenide devices. It was recognized that these efforts, if successful, had the potential for enhancing the attractiveness of this compound semiconductor material for use in making practical devices of commercial importance such as FETs and opto-electronic integrated devices.